This invention relates to altimeters for aircraft, and more particularly to a digital encoding altimeter for measuring changes in altitude of an aircraft and converting such altitude changes to digital signals which are in turn converted to a parallel digital code acceptable for transmission of aeronautical altitude information, such as the ICAO international code.
In the prior art, altitude encoders have been of the absolute or direct reading type. Absolute encoders provide a binary output signal of several bits which is representative of shaft position any time the encoder is turned on. This output is different for different angular positions of the shaft. As an example, an encoding altimeter of the absolute type has eight to 10 code tracks and eight to ten optical sensors to read these tracks. These encoders must be able to resolve 150 - 600 different angular positions in one revolution, and because the code discs are only about 5 centimeters in diameter, it is apparent that errors of greater than a few tenths of a millimeter in either sensor alignment or track registration will cause errors in the output. Moreover, the clearances between the encoder disc and sensors must be on the order of a few tenths of a millimeter, and this close fit makes it possible that shock or vibration, always prevalent in aircraft, will cause some misalignment which will result in erroneous output signals. A much more serious consequence of misalignment occurs if the disc, which is driven by the altimeter mechanism, comes into contact with the nearby sensors. This frictional drag can easily cause greater torque than the mechanism can overcome. The result is a "stuck" altimeter. If, as is usually the case, the pilot is also using this altimeter for an altitude reference, an extremely dangerous situation is created. Such encoding altimeters of the absolute or direct reading type are shown in U.S. Pat. Nos. 3,750,473 and 3,808,431. Other altitude encoding devices are shown in U.S. Pat. Nos. 3,546,470, 3,513,708 and 3,750,474.
Accordingly, one primary feature of the present invention is to provide an incremental altitude encoder which provides the same or repetitive output signal each time the altimeter indicating shaft is moved through a predetermined angle.
Another feature of the present invention is to provide a digital altimeter encoder which includes a counter which is presettable to a predetermined digital count representative of a preselected, altitude function, and to which the incremental signal provided by the rotation of the shaft is added or subtracted to provide an output which is representative of altitude.
Yet another feature of the present invention is to provide a digital altitude encoder which is inherently more accurate, simpler in construction, and less critical in the design of the mechanical parts of the rotating disc and spaced sensors.
Still another feature of the present invention is to provide a digital altitude encoder which utilizes a single track code disc which makes only a predetermined number of transitions per revolution of the shaft, and hence can tolerate errors in alignment of several millimeters, and further provides for clearance between the sensor and disc of several millimeters.